A majority of engine air intake manifolds have a fixed volume plenum with fixed length runners tuned for a particular application, such as low rpm torque, midrange torque, or high rpm torque.
Variable geometry intake manifolds provide more than one tuned frequency to optimize engine performance over a wider range of engine rpm. One such variable geometry intake manifold utilizes a pair of plenums connected together by differing length communication passages to vary the tuning and effective volume of the plenums. The communication passages are controlled by separate valves, which selectively open or close their respective communication passages to vary flow between the plenums and thereby alter the effective volume of the plenums. By varying the effective volumes, the resonant frequencies of the manifold can be extended to optimize airflow through the plenums and obtain desired volumetric efficiencies at various engine rpm.
Another type of variable geometry intake manifold uses a fixed volume plenum connected to short and long runners. This manifold also uses multiple valves to vary the geometry of the manifold by switching between two runner lengths. When switched to the longer runner length, the resonant frequency of the intake manifold decreases. This provides additional engine torque at lower engine speeds. When switched to the shorter runner length, the resonant frequency of the intake manifold increases thereby increasing the engine speed where maximum volumetric efficiency occurs. This provides additional engine horsepower at higher engine speeds.